Most commercially available flat panel displays are currently made on glass substrates. Glass substrates offer many advantages in manufacturing displays since they are compatible with many process technologies. From a user perspective, glass substrates have many disadvantages: they are heavy, rigid, prone to breakage from mechanical shock, and difficult to conform to forms that are not flat. By using flexible substrates instead of sheet glass, these issues are significantly reduced or eliminated completely. For this reason, flexible displays and electronics (RFID tags, etc.) are highly desired for military applications where user environments are harsh and reducing power and weight and improving ruggedness are desired characteristics.
There is a large commercial industry already established based on liquid crystal display (LCD) technology. This display architecture is not desirable for portable military applications; LCDs generally require a backlight, and the use of color filters to generate color significantly reduces the power efficiency. Standard cold-cathode fluorescent lamp (CCFL) edge light backlight technology is counter to the flexible display concept, although new LED edge lighting may improve the situation. Flexible reflective or emissive displays such as organic LED (OLED) technologies are preferred. Unfortunately, even these display technologies require an active circuit backplane in order to achieve the necessary uniformity, lifetime, brightness and efficiency.
The first attempts at putting active circuits onto flexible substrates were to modify existing processes already developed for glass substrates. Amorphous silicon (a-Si) thin-film transistors (TFTs) are widely used in the active matrix liquid crystal display (AM-LCD) industry. Production of a-Si TFT-LCDs has already exceeded Gen 7 glass substrate sizes (1870×2200 mm). Unfortunately, what works for LCD technology, which is a voltage-driven technology, does not work well for OLED technology, which is a current-driven technology. Although a-Si TFTs work wonderfully for LCDs, the mobility is too low and the stability is too poor for OLED applications. Table 1 shows typical requirements for active circuit elements for both AM-LCD and AM-OLED applications.
TABLE 1Comparison of active circuit elementrequirements of LCD and OLED applications.AM-LCDAM-OLEDIOFF (A)<2 × 10−13<10−12ION (A)>2 × 10−7>10−6ION/IOFF>106>106Vth (V) <2 <2S (V/dec) <0.5 <1.0τ (switching)N/A>200 ns
To improve the mobility of the a-Si TFTs, they are subjected to a heat-treatment using a laser beam that anneals the Si layer to form polycrystalline Si. The material from this process is generally referred to as low-temperature poly-Si, or LTPS. LTPS TFTs have higher mobilities but also tend to have larger variations in threshold voltage (Vth) that result in display non-uniformity (generally referred to as mura). This is especially true when this process is transferred to flexible substrates. Although a-Si transistors have been successfully processed (N. D. Young, et al., “Thin-Film-Transistor and Diode-Addressed AMLCDs on Polymer Substrates,” J. SID, Vol. 5-3, pp. 275-281, 1997) directly on plastic substrates, the technology is still in a research phase. Problems with a-Si and LTPS technologies include the difficulty of forming a high-quality layer at temperatures sufficiently low to prevent the plastic substrate from degrading, which results in reduced transistor performance (G. H. Gelinck et al., “Rollable QVGA AM Displays Based on Organic Electronics;” SID '05 Digest, p. 6, 2005), The flexibility and expansion/shrinkage of polymer films as a result of large process temperature swings also introduces problems with mask, alignment in the photolithography process and in the handling of polymer films. High temperature processes shrink the polymer film, often non-uniformly, leading to warping of the film and contribute to misalignment of subsequent layers. Stresses from deposited layers treated at high temperature lead to curling of the film.
There have been several approaches to address these issues. In a first approach, substrate transfer technologies, in which silicon-based transistors are first manufactured on rigid substrates suffered from unreliable contacts (S. Inoue et al., IEEE Trans. Electron Devices, Vol. 49, pp. 1353-60, 2002). In a next approach, carbon nanotube and other semiconducting nanowires and nanoparticles were used by several groups in an attempt to make a nanoparticle-based TFT technology. Early work at the Naval Research Lab (NRL) (E. S. Snow et al., “Random Networks of Carbon Nanotubes as an Electronic Material,” APL., 82, p. 2145, 2003) and then followed up by work at Applied Nanotech, Inc. (ANI) (J. P. Novak et al., “Flexible Carbon Nanotube Thin-film Transistors,” IDW/AD '05, p. 257, 2005) demonstrated that CNT-based TFT can be deposited using printing techniques. ANI also demonstrated that the mobility of the material and the current through the device in the on-state were sufficient to drive large LCD segmented display pixels and LED devices, but that the on/off current ratio was only 104. To improve this ratio would require an ink containing only semiconducting single-wall nanotubes. Many groups are working on this and if successful can lead to a significant breakthrough for this and other printable microelectronic applications. Rice University has demonstrated significant progress using an electrophoresis technique (Haiqing Peng et al., “Dielectrophoresis Field Flow Fractionation of SWNT,” JACS Comm. web Jun. 9, 2006) to create an ink of semiconductor-enriched CNTs. This enriched solution is part of the approach used in the present invention.
Another approach would be nanowires of Si, Ge or other semiconducting compounds. Charles Lieher and the company Nanosys have demonstrated TFTs made of Si nanowires and CdS nanoribbons on a Si substrate and PEEK polymer sheet (X. Duan et al., “High-performance TFTs using Semiconductor Nanowires,” Nature, 425, 274, (2003)). These devices demonstrated excellent TFT performance on both Si and polymer substrates (Vth˜3.0V; on/off ratio greater than 105 on polymer substrates; subthreshold swing of 500-800 mV/decade, hole mobilities estimated at 123 cm2/Vsec), The channel length of these devices was 5 μm. Because of the relatively large channel length, these excellent properties were achieved only when the Si nanowires were aligned. Little change in the characteristics were observed when the polymer substrate was bent to a radius of 55 mm, thus demonstrating that there was sufficient adhesion and flexibility of the nanowires to allow flexing of the substrate. Use of Si and GaN nanowires will also be used in the present invention along with Inkjet printing to deposit randomly orientated nanowires and ac-biased electrophoresis to deposit aligned nanowires.
Most importantly, other than the fabrication of the CNT or the semiconducting nanowires, the other process steps were truly low temperature and fully compatible with polymer substrates.
Polymer semiconductors approaches are being investigated by several companies (Polymer Vision. Plastic Logic, etc.) and research centers (Kyung Hee University. University of Michigan, etc.). There is now a wide variety of these materials. Table 2 shows the properties for a few of the more widely-used materials. Pentacene is the most popular of these materials. It can be deposited either by vacuum evaporation through a shadow mask or printed via solution. The properties of the material are much better if deposited through vacuum evaporation. Polymer semiconductors suffer from high threshold field (30V) and low mobility (1 cm2/Vsec, similar to a-Si), but have demonstrated excellent uniformity. Process temperatures are less than 130° C., compatible with polymer substrates. Table 3 summarizes the display prototypes using organic TFTs (I Jang and S. H. Han, “High-Performance OTFTs on Flexible Substrates,” SID 06 Digest, p. 10, 2005). E-paper and LCD were made with organic TFTs matrix arrays and AM-OLEDs were made with dot patterns. The AM-OLED suffered from significant brightness non-uniformity, mainly due to the grain size distribution of the polycrystalline organic semiconductors. On the other hand, LCD and E-paper displays need only high on/off current ratio and are generally immune to differences of TFT current in the on state.
TABLE 2Organic SemiconductorTypical TFT PerformancePentaceneμ = 1 cm2/VsIon/Ioff > 106Poly(3-hexylthiophene)μ = 0.1 cm2/VsIon/Ioff > 104Polyfluorene-based polymerμ = 0.1 cm2/VsIon/Ioff = 106Regioregular poly(thiophene)μ = 0.1 cm2/Vs(XPS)Ion/Ioff = 106
TABLE 3ResearchOrganizationApplicationSemiconductorSpecificationPlastic LogicE-PaperPolyfluorene-based60 × 80 pixels on(UK) & E-InkpolymerPET(USA)(ink-jet printing)PhilipsE-PaperPentaceneQVGA on PEN(Netherlands)(solution-process)HitachiLCDPentacene1.4” 80 × 80 RGB(Japan)on glassERSO/ITRILCDPentacene64 × 128(Taiwan)on plasticSamsung Elec.LCDPentacene15” Full color XGA(Korea)on glassNHKOLEDPentacene4 × 4 pixel on PC(Japan)PioneerOLEDPentacene8 × 8 pixels(Japan)on glass
In summary, for emissive display technologies that can be fabrics led on plastic, such as OLED, TFT devices with high on-current levels that, are uniform from pixel to pixel are required. Although a-Si and LTPS TFT approaches work well for glass substrates, they require process temperatures that are too high for standard polymer substrate materials. This leads to poor TFT performance, warping and curling of the substrate materials and misalignment of patterns from level to level.
For polymer substrates, it would also be a great advantage if the active matrix array could be fabricated using printing techniques, and avoiding photolithography processing as much as possible. Printing can be performed roll-to-roll and over surfaces as large as a billboard. Since printing is an additive process, fabrication costs can be reduced as a result of lower material costs and fewer processing steps.